Colloidal solution comprising silver metal particles and a silane derivative

ABSTRACT

The invention pertains to a composition comprising silver metal particles and an additive, characterized in that the additive is a silane derivative comprising at least one methyl group and at least one alkoxy group. Preferably, the silane derivate is methyl trimethoxysilane, methyl triethoxysilane, or a mixture thereof. The compositions can be used to make thermal resistant conductive silver-containing layers, for instance for use in AMLCD.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International ApplicationPCT/TBO2/04461, filed Oct. 24, 2002, which claims priority from EuropeanApplication 01204334.5, filed Nov. 13, 2001.

BACKGROUND OF THE INVENTION

The invention pertains to a composition comprising silver metalparticles and an additive, to thermally resistant conductive layers, toan active matrix liquid crystalline display (AMLCD) comprising the saidlayers, and to the use of said composition in the manufacture ofsilver-containing layers in an article.

Compositions comprising silver particles with or without otherconstituents, are well known in the art. For instance, in EP 826,415 amethod for preparing a silver sol has been disclosed. These silver solsare used for making a conductive film on a substrate. By spin-coatingthe sol onto a substrate, such films are made and these are then heatedat 150° C.

In EP 276,459 a method for manufacturing cathode-ray tubes wasdisclosed. Antistatic silicon dioxide films were prepared that containedsmall amounts of (among others) silver. To a solution or a colloidalsolution containing metal particles a cationic or anionic surfactant wasadded as an additive for improving the stability of the solution, afterwhich an antistatic film was produced by a spraying, dispensing, ordipping method of this material onto a substrate, followed by heating at200° C. for 15 min.

It was found that silver-containing layers, particularly layers thatcontain 80 vol. % or more silver, cannot be heated above 250° C., andpreferably not above 200° C. without serious appearance of irreversibleexfoliating and/or creep phenomena. Creep is a process wherein the filmdeteriorates into a plurality of small sections containing silver andsection therein between containing no silver. On creep the surfaceroughens, which means that the mirror-like appearance of thesilver-containing layer disappears. Silver-containing layers (or films)that have undergone exfoliation and/or creep no longer have a lowresistivity, and because of the low conductivity have become unsuitablefor most of the applications that require a resistivity of less than 6μΩ.cm (microOhm.centimetcr). Since silver-containing layers are usuallymade of compositions also comprising organic materials such as organicbinder materials, and because only relatively low temperatures (lessthan 250° C., preferably less than 200° C.) can be used for makingsilver-containing layers with sufficient conductivity, such layersusually contain amounts of organic materials that are not sufficientlyremoved at those temperatures. Also in further processing steps hightemperatures (higher than 250° C.) may be necessary. For instance,flitting a CRT-cone to a screen occurs at 450° C. To make it possible touse silver at such high temperatures, a polymeric binder and frit glassparticles can be mixed into a silver paste. However, the presence of theglass particles reduces the conductivity considerably. Furthermore,since these added particles have sizes in the micron range, such mixedpastes are no longer suitable to make silver-containing layers thinnerthan 1 μm. Thin layers of well-conducting silver on an insulatingsurface, such as on Coming®1737 glass, which are used for active matrixliquid crystal displays (AMLCD), or on glass or any other substrate foruse in infrared reflective stacks, can be made by using an electrolesssilver process, but the maximum temperature in the further reactionsteps is limited to 250° C. At that temperature Si₃N₄ deposition takesplace, and moreover, the electroless process is commercially not desiredanyway since it is a slow non-equilibrium process with a short bath potlife.

There is a considerable need for a method of making thin conductivesilver-containing layers that can be heated at temperatures higher than250° C. without losing its conductivity and mirror-like appearance.

SUMMARY OF THE INVENTION

The invention pertains to a composition comprising silver metalparticles and a silane derivative. The compositions can be used to makethermal resistant conductive silver-containing layers, for instance, foractive matrix liquid crystalline displays.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the resistivity of spin-coated silver-containing layers asa function of curing temperature for colloidal silver/methyltrimethoxysilane at different volume fractions of methyltrimethoxysilane. according to an aspect of the instant invention;

FIG. 2 shows the layer thickness of spin-coated silver-containing layersas a function of curing temperature for colloidal silver/methyltrimethoxysilane at different volume fractions of methyltrimethoxysilane, according to another aspect of the instant invention;

FIG. 3 shows the resistivity of spin-coated silver-containing layers asa function of curing temperature for colloidal silver/methyltrimcthoxysilane at different volume fractions of methyltrimethoxysilane, according to another aspect of the instant invention;and

FIG. 4 shows the surface profile of spin-coated silver-containinglayers, cured at 250° C. for 1 hour and subsequently heated to 500° C.,for different volume fractions of methyl trimethoxysilane, according toanother aspect of the instant invention.

DETAILED DESCRIPTION OF THE INVENTION

It was now surprisingly found that compositions comprising silver metalparticles and an additive, which is a silane derivative comprising atleast one methyl group and at least one alkoxy group, are very stableand can be applied onto a substrate and heated to temperatures as highas 450° C., usually as high as 700° C., and in many instances even ashigh as 1000° C. It was further found that the resistivity ofsilver-containing layers of 200 nm thickness made of these compositionsat these high temperatures remained as low as 2.5 to 6 μΩcm, whichvalues are completely comparable with the resistivity ofsilver-containing layers that are obtained by electroless depositingsilver at 250° C., which was found to be 3–4 μΩcm. A further advantageof the present invention is that the silver-containing layers showimproved adherence to inorganic substrates such as glass.

Moreover, the silver-containing layers do not show aging, i.e. themirror-like appearance of the silver-containing layer does not visuallychange and does not show increase of resistivity over a period of atleast 1 year.

It was now surprisingly found that compositions comprising silver metalparticles and an additive, which is a silane derivative comprising atleast one methyl group and at least one alkoxy group, are very stableand can be applied onto a substrate and heated to temperatures as highas 450° C., usually as high as 700° C., and in many instances even ashigh as 1000° C. It was further found that the resistivity ofsilver-containing layers of 200 nm thickness made of these compositionsat these high temperatures remained as low as 2.5 to 6 μΩcm, whichvalues are completely comparable with the resistivity ofsilver-containing layers that are obtained by electroless depositingsilver at 250° C., which was found to be 3–4 μΩcm. A further advantageof the present invention is that the silver-containing layers showimproved adherence to inorganic substrates such as glass. Moreover, thesilver-containing layers do not show ageing, i.e. the mirror-likeappearance of the silver-containing layer does not visually change anddoes not show increase of resistivity over a period of at least 1 year.

These profitable properties make silver-containing films of thecomposition of the invention very suitable for application in AM LCD,passive integration, high temperature-resistant reflective layers (suchas used in CRTs having a tracking structure instead of a shadowmask,also referred to as Fast Intelligent Tracking-FIT), and in reflectivedisplays. Due to their enhanced stability on ageing, it is alsoadvantageous to use these silver-containing laycrs in low temperatureapplications, such as its use for making mirrors for applications whereageing is a problem.

Preferably, the silane derivative is methyl trialkoxysilane, wherein thealkoxy moieties have 1 to 4 carbon atoms. Preferred alkoxy groups aremethoxy and ethoxy groups. Most preferred silane derivative is methyltrimethoxysilane (MTMS), methyl triethoxysilane (MTES), or a mixturethereof. Preferably, the composition comprises <20 vol. % of the silanederivative, most preferably <10 vol. %. At concentrations higher than 20vol. % the conductivity decreases as the result of the low silvercontent.

The most common composition of the invention is a colloidal silver solcomprising the silane derivative. These sols are very stable and have along pot life. Application of these sols onto a substrate can simply beperformed in the manners known in the art, for instance by aspin-coating or printing process.

The composition is usually brought onto a substrate, usually aninorganic substrate such as glass to form a thermal resistant conductivesilver-containing layer. These layer can be used, i.a. in an activematrix liquid crystalline display.

In general the compositions are also very suitable for makingsilver-containing layers for use in a device wherein thesilver-containing layer is exposed to a temperature of 200° C. orhigher. When these silver-containing layers comprising the silanederivative additive are exposed to such temperatures the additive,disappears completely or partially from the silver-containing layerunder decomposition to silicon dioxide and/or silsesquioxane, whichdecomposition products then form a layer adjacent to thesilver-containing layer. The invention therefore also pertains to athermal resistant conductive layer comprising a silver-containing layerand a layer comprising at least one of silicon dioxide andsilsesquioxane. Such thermal conductive layers can be applied to anarticle, such as an AMLCD or in IR reflective stacks.

The invention also relates to a specularly reflecting silver layerobtainable by curing a layer formed of a composition in accordance withthe invention. The advantages of such layers has been describedhereinabove and will be elucidated further with reference to theexamples described below.

The invention also relates to a substrate provided with a silver layerhaving a thickness less than 1 μm and capable of specularly reflectingincident light if exposed or after having been exposed to temperaturesof 250° C. or more.

Using the compositions in accordance with the invention such silverlayers can be made using conventional wet deposition methods such asspin-coating or printing methods such as ink jet printing.

The invention is illustrated by the following examples.

EXAMPLE 1

Colloidal silver (6 g, ex Merck) was added to 20 g of water and rolledfor one night on a roller conveyer and the sol dispersion was filteredover a 200 nm filter. A composition containing

-   40 g methyl trimethoxysilane (MTMS)-   0.86 g tetraethoxysilane (TES)-   32 g water-   4.5 g ethanol-   0.14 g glacial acetic acid    was hydrolyzed for 48 h. 0.09 g of this hydrolyzed mixture were    added to 4 g of the above silver sol dispersion under continuous    stirring, to give a silver-containing layer comprising 10 vol. % of    MTMS (assuming a density of 2 g/ml for fully condensed MTMS). In a    similar manner silver sols comprising 5%, 1%, and 0% of MTMS,    respectively were prepared. Silver-containing layers were    spin-coated onto 1% HF cleaned glass plates. The spin-coated glass    plates were cured at 250° C. for 30 min, followed by a further heat    treatment for 30 min at 350, 450, 500, and 550° C. The results    obtained with these silver-containing layers with respect of    resistivity and layer thickness are depicted in FIG. 1 and FIG. 2.

EXAMPLE 2

MTMS was added to a dispersion of silver colloids in water (ex NipponPaint). The amount of MTMS leads to the formation of silver-containinglayers with 0, 5, 10, 20, and 50 vol. % of CH₃—SiO_(3/2) (assuming adensity of 2 g./ml). The layers were deposited deposited onto a Coming1737 substrate by spin-coating. Afler heating at a temperature withinthe range 200 to 700° C. (high heating rates), the resistivity wasmeasured using a four-point probe. The results arc given in FIG. 3.Although good conductivity could be obtained after low temperatureprocessing of the MTMS free (0 vol. %) sample, the adhesion of thesilver to the substrate was found to be inferior.

The amount of MTMS added also influences the heating rate that can beapplied. Samples were cured at 250° C. for 1 h, and subsequently heatedto 500° C. (dwell time 1 h) at the indicated heating rates. The resultsare given in FIG. 4.

EXAMPLE 3

44.6 mg of MTMS were added to 10 ml of water and homogenized. To thissolution 3 g of colloidal silver (ex Merck) were added and the mixturewas stirred overnight with a roller bench. The solution was spin-coatedonto a glass substrate, yielding silver-containing layers comprising 5vol. % of MTMS.

EXAMPLE 4

58.4 mg of MTES (methyl triethoxysilane) were added to 10 ml of waterand homogenized. To this solution 3 g of colloidal silver (ex Merck)were added and the mixture was stirred overnight with a roller bench.The solution was spin-coated onto a glass substrate, yieldingsilver-containing layers comprising 5 vol. % of MTES.

1. An active matrix liquid crystalline display (AMLCD) comprising asilver containing layer adjacent to a silsesquioxane layer.
 2. A methodfor making an active matrix liquid crystal display, comprising: a)providing a composition, wherein the composition comprises silver metaland a silane derivative, wherein the silane derivative is amethylalkoxysilane; b) applying the composition to a substrate; and c)heating the composition applied in said step b), wherein said heatingthe composition in said step c) results in the completion of thefollowing steps i)–iii): i) forming a first layer on said substrate,said first layer comprising silver metal; ii) decomposing the silanederivative to form silsesquioxane; and iii) forming a second layeradjacent to the first layer, wherein the second layer comprisessilsesquioxane, silicon dioxide, or a mixture thereof.
 3. The method ofclaim 2, wherein the methylalkoxysilane is methyltrimethoxysilane,methyltriethoxysilane, or a mixture thereof.
 4. The method of claim 2,wherein the composition provided in said step a) comprises <10 vol. % ofthe silane derivative.
 5. The method of claim 2, wherein said step c)comprises heating the composition on the substrate at a temperature of250° C. or more.
 6. The method of claim 2, wherein the first layer is aspecularly reflecting layer.